In the float glass process in which a bath of molten tin or tin alloy is used to support an advancing ribbon of glass, oxygen is a major contaminant. Oxygen is believed to be responsible directly or indirectly for reducing glass quality in several ways, giving rise to poor bloom grades, CO bubble, tin speck faults and for tin pick-up faults.
In the float glass process the bath atmosphere is controlled, generally by maintaining an atmosphere of nitrogen and hydrogen. In practical terms it is impossible to exclude oxygen completely from the bath; oxygen can enter the bath by various pathways, such as through the exit seal, through leaks in windows and side seals, as a contaminant of the atmosphere supply, or with the glass itself as dissolved oxides (e.g. SO.sub.2 and H.sub.2 O). A series of interactions then take place with hydrogen in the atmosphere, with the tin, and with the glass itself. It is not economic to attempt to reduce the contamination below a particular level. Ingress can, however, be reduced to a level at which its deleterious effects are of little or no consequence.
It is clearly important to know the amount of oxygen present so that if this rises, appropriate steps can be taken to prevent further contamination, whilst the glass subject to contamination and the level of contamination can be readily identified.
The oxygen content of the tin can be measured by conventional analytical techniques; by removing a sample, reacting with carbon under vacuum and measuring the carbon monoxide released. This is however a lengthy procedure which demands great skill and care if the required degree of precision is to be achieved; as a result, this measurement is rarely performed.
The chemical state of the bath with respect to oxygen contamination can also be monitored on a regular basis using indirect means such as an atmosphere extractive technique, measurement of the tin count, or measurement of the bloom grade. The extractive technique indicates the level of contamination of the atmosphere but does not necessarily say anything about the level of contamination in the tin. Further, being an extractive technique, the sample lines are prone to blockage. The tin count and bloom grade are measurements made on the glass, and are indicative of the amount of tin present in the surface, which is directly related to the level of oxygen contamination. Since these tests are carried out on the product there is necessarily a time delay and the results give little indication of the distribution of contamination within the bath.
A known technique for measuring the oxygen content of the molten tin on an in situ basis uses a measuring probe located in the bath so as to extend into the molten tin. This probe is the subject of U.S. Pat. No. 3,625,026. The probe comprises a tubular body of zirconia which has been doped to induce conductivity to oxygen ions and which thereby constitutes a solid electrolyte. Electrical connection is made to the inside of the tube, and directly to the molten tin which as a conductor constitutes an electrical connection to the outside of the tube. A galvanic cell is thereby effectively set up, resulting from the oxygen concentration internally of the tube which is separated from the oxygen concentration in the molten tin by the solid zirconia electrolyte. The cell emf is indicative of the oxygen concentration at the outer side of the probe. By supplying a gas of constant oxygen concentration to the inside of the tube an absolute value for the oxygen concentration externally of the tube can be determined.
Various problems exist with this probe. The probe body of zirconia is particularly fragile, and is sensitive to thermal shock arising on insertion of the probe into the molten tin, which may typically be at 700.degree. C. Zirconia has a high thermal coefficient of expansion, such that on this initial insertion, considerable stresses are set up within the zirconia body, rendering the probe liable to fracture. There is a further thermal effect which arises on more prolonged usage which can cause the probe to fracture. The stabilisation of the zirconia by the addition of dopants causes the zirconia to take a particular crystalline form (specifically, a cubic tetragonal form). Although this form is stable at the temperature of the molten tin, at lower temperatures, below about 400.degree. C., the stable form of zirconia is a different crystalline form (specifically, a cubic monoclinic form), and on prolonged usage a transition to this form will occur. A substantial temperature gradient exists along the length of the probe when in use, such that the temperature conditions in which the second crystalline form is the stable form are generally found at a region of the probe remote from the molten tin. This change of crystalline structure involves a volume change, such that a junction region between the two forms of zirconia will constitute a particular site of stress at which the zirconia body is liable to fracture.